Generally, there exists two types of photovoltaic panels used for capturing solar energy for conversion to electrical power. One type of solar panel is crystalline silicon wafer panel while the other type is the so-called thin film photovoltaic panels (TFPP). As the name suggests, a crystalline silicon wafer panel design employs crystalline silicon wafers connected together and embedded in a laminating film. The laminating film and the wafers embedded therein are typically sandwiched between two lites, or panels, of glass, a polymeric material or other suitable materials.
The TFPP design, which is of primary interest herein, employs one of amorphous silicon, cadmium-telluride (Cd—Te) or copper-indium-diselenide, CuInSe2 (commonly referred to as “CIS”), or a similar semiconductor material such as mentioned below, which is deposited on a substrate in a thin film. These thin film photovoltaic materials are typically deposited in a thin film on a substrate by a method such as sputter coating, physical vapor deposition (PVD) or chemical vapor deposition (CVD). The photovoltaic material of the TFPP is often covered by a sputtered layer of aluminum, which acts to protect the underlying structures. The individual photocells are then typically formed by a laser etching process, and are connected together by suitable circuitry, such as a buss bar. The buss bar transfers the electrical current output from the photocells to a storage device such as a battery or directly to a load. To complete the construction, a laminating adhesive is applied over the photovoltaic material, associated circuitry, and any protective layer which is present, and a backing panel is then applied. The backing panel is typically glass, but may be metal, a composite or a plastic material.
The circuitry, such as a buss bar which collects the electrical current generated by the solar panel must be connected by wiring to a suitable storage device, such as a battery or directly to a load. Such wiring may be referred to as a “module wire” or “module lead”. The module wire must exit the solar panel at some point. Additional adhesive or sealant material is needed to seal around the module wire exiting the solar panel. The adhesive used for sealing around module wires may be the same as, or may differ from, the laminating adhesive used to attach the backing material to the solar panel.
TFPP's are used outdoors, and so are exposed to the elements, including wind, water and sunlight. TFPP's are deleteriously affected primarily by moisture which may permeate into the panel, reaching the electrical connections or the photovoltaic materials. Water penetration into solar panels has been a long-standing problem. Thus, various attempts have been made to reduce the moisture vapor transmission rate (MVTR) of the laminating film. Solar panels may also be deleteriously affected by wind and sunlight, which may result in failure of the adhesive layer. Wind can cause physical damage such as stresses on the adhesive layer while sunlight can result in heating of the solar panel and exposure to ultraviolet (UV) radiation. Operating temperatures of solar panels have been measured as high as 110° C.
A commonly used laminating adhesive is ethylene vinyl acetate (EVA). The EVA is applied as a film to the photovoltaic material. The film is formulated to contain a peroxide, which is designed to crosslink the EVA. The EVA is then cured in place on the solar panel by application of heat or radiation, which causes the peroxide to crosslink the EVA. Crosslinked EVA provides high strength at room temperature and adequate strength at operating temperatures, but suffers from a relatively high MVTR. The MVTR can be in the range of 40-50 grams of H2O/meter2/per day measured 37 C.
The typical manufacturing process for thin film photovoltaic panels requires cutting the laminating film to an appropriate size and includes removing any portion of the film that may cover electrical connections. The film is then sandwiched between the substrate containing the photovoltaic semiconductor material and a backing material, typically glass. This structure is then heated and pressed to affect a cure and cross-link the EVA. A vacuum may be applied to remove air trapped between the film and the substrate. The total time to assemble and heat and cure each solar panel is significant, approaching 15 to 20 minutes per panel. By reducing the manufacturing time, the efficiency of TFPP manufacturing can be greatly improved. The current manufacturing technique is necessitated because of the need to cure EVA.
The present invention improves manufacturing efficiency by changing the application technique and cure chemistry of the laminating adhesive.